Introduction to receptors

Receptors are specialised proteins that recognise a particular signal and help a cell respond. The signal may be a hormone, neurotransmitter, drug, toxin, ion, or a physical change such as stretch. When a signal binds to its receptor, the receptor changes activity and starts a response inside the cell.

The response can include opening an ion channel, changing the activity of an enzyme, altering gene expression, or releasing a chemical messenger. A single signal can have different effects in different tissues because the tissues may carry different receptor types and use different signalling pathways.

How receptors work

One useful classification groups receptors by the way they transfer information. Some act inside the cell, while others sit in the cell membrane and communicate with the interior.

Intracellular receptors

Intracellular receptors are found in the cytoplasm or nucleus. Their signals can cross the cell membrane, as happens with many steroid and thyroid hormones. After binding, the receptor–signal complex can interact with DNA and influence which genes are turned on or off. These effects often develop more slowly but can last longer than changes caused by an ion channel.

Ion-channel receptors

Some membrane receptors are directly linked to ion channels. When a signal binds, the channel opens or closes and allows selected charged particles to cross the membrane. This rapidly changes the electrical state of the cell. Nicotinic acetylcholine receptors, many glutamate receptors, and GABA_A receptors are examples of ionotropic receptors.

The effect depends on the ion and the direction of its movement. Sodium entry can make a neuron more likely to send a signal, while chloride entry commonly makes it less excitable. These receptors are especially important for quick communication between nerve cells and muscles.

G protein-coupled receptors

G protein-coupled receptors, often called GPCRs, cross the cell membrane seven times. A signal binding outside the cell changes the receptor’s shape and activates a G protein on the inside. The G protein then influences enzymes or ion channels through an intracellular signalling cascade.

Three familiar pathways are named for the G protein involved. G_q commonly activates phospholipase C and increases intracellular calcium. G_s commonly stimulates adenylate cyclase and raises cyclic AMP. G_i commonly reduces cyclic AMP production. The final response depends on the receptor, the cell, and the proteins available in that cell.

Tyrosine kinase pathways

Some receptors contain an enzyme domain called tyrosine kinase. Binding a signal activates the receptor and adds phosphate groups to selected proteins, beginning a chain of events that can influence metabolism, growth, and cell division. The insulin receptor is a well-known example.

Other receptors do not contain their own kinase domain but are connected to a separate kinase inside the cell. This arrangement is used by several cytokine and growth-related signals. The JAK–STAT pathway is one example: activated proteins pass information to the nucleus, where gene activity can change.

Physiological groups of receptors

Receptors can also be grouped by the kind of information they detect or the system in which they operate.

Hormone receptors

Hormone receptors detect chemical messengers released into the body. Depending on the hormone, the receptor may be inside the cell, connected directly to an ion channel, linked to a G protein, or associated with a kinase pathway.

Autonomic receptors

Autonomic receptors help regulate automatic functions such as heart rate, digestion, pupil size, and gland activity. Cholinergic receptors respond to acetylcholine, while adrenergic receptors respond to adrenaline and noradrenaline. Their effects vary according to the receptor subtype and the tissue involved.

Immune receptors

Immune receptors help cells recognise microbial components, damaged cells, and signals released during inflammation. Toll-like receptors, NOD-like receptors, and lectin-related receptors can activate intracellular pathways that coordinate an immune response.

Muscle receptors

Muscle spindles detect changes in muscle length, while Golgi tendon organs detect tension where muscle meets tendon. Their feedback helps the nervous system adjust force, posture, and movement.

Sensory receptors

Sensory receptors translate physical or chemical changes into signals for the nervous system. Mechanoreceptors detect touch, pressure, vibration, and stretch. Thermoreceptors respond to temperature, nociceptors respond to potentially damaging stimuli, photoreceptors detect light, and chemoreceptors detect substances involved in taste, smell, and internal regulation.

Read more about sensory receptors

Summary

Receptors allow cells and nervous systems to detect what is happening inside and outside the body. Their location and signalling mechanism determine how quickly a response begins and what kind of response follows.

Some receptors change gene activity, some open ion channels, and others use messenger cascades involving G proteins or kinases. Grouping receptors by mechanism and function makes this large subject easier to navigate.

Further reading

Continue with neurotransmitters and receptors, sensory receptors, or reflex actions to see how signals become perception and movement.